YAKUTAT BAY GLACDEBS 31
plete stagnation, and in the case of portions of the piedmont bulbs actual stagnation. In some cases the outer portions of the bulbs become completely motionless. Moreover, the tidal glaciers terminate farther back in the mountains than the other types, for the sea eats into them more rapidly than the rams and warm air can melt away those glaciers which end on the land.
As a result of these peculiarities there are among these glaciers distinct differences in the phenomena attending the destruction of their outer portions. From the steep faces of the crevassed tidal glaciers icebergs are constantly thundering as they fall into the sea or rise from the submerged base of the ice cliff. Both ice and debris are borne away from the glacier front, the ice to melt and the debris to be deposited at a distance. Being back among the mountains, in a cooler climate, there is less melting of the surface of the tidal glaciers than of those that project as piedmont bulbs beyond the mountain face. There is consequently less water emerging from these tidal glaciers, and that which does issue comes out mainly from ice tunnels beneath the sea. Because of the rapidity of ice motion, the relative slowness of melting, and the active discharge of icebergs, the tidal glaciers have small amounts of debris upon their surfaces. Only along the lateral margins, where the motion diminishes, where iceberg discharge is either checked or entirely stopped, and where the debris supply is great, are there extensive fields of rock fragments accumulated on the surface as a result of ablation.
Those glaciers which terminate in their mountain valleys are sometimes covered byfields of debris, but in other cases have little. The Black Glacier, for example, is completely buried beneath morainic d6bris in the lower third of its valley portion; Fourth Glacier has some de'bris, but not much, while Hidden Glacier is one of the freest from debris cover of all the glaciers of the region; but this is in part, if not entirely, due to the fact that its real terminus was buried (up to 1906) beneath an outwash gravel plain. The piedmont bulbs and piedmont glaciers, on the other hand, are always either partly or completely buried beneath broad fields of morainic waste. This is due to the fact that these are large glaciers, with much incorporated rock debris, which spread out beyond the mountain front, assume a condition of stagnation or semi-stagnation, and then slowly waste away in the cool temperature, rainy climate of the Alaskan coast. Ablation then concentrates rock fragments on the ice surface, giving rise in some cases to areas of many square miles of barren rocky de'bris, which has been called an ablation moraine.* It consists mainly of de'bris avalanched down upon the glacier within the mountains, incorporated in the ice mass, and near the glacier terminus concentrated at the surface by the wasting, or ablation, of the ice. In the more stagnant portions of the glacier the ablation moraine becomes so thick that it serves to so protect the glacier that further melting proceeds very slowly. Then the soil assumes such stability that plants may find a foothold. On the inner margins of this portion of an ablation moraine willows and alder bushes are found as scattered individuals or in clusters; but on the outer margin the ablation moraine may become so stable that forests of cottonwood and spruce grow, with trees a half century or more old.2 The Malaspina, Lucia, and Atrevida Glaciers each furnish illustrations of this condition.
i Tarr, R. S.r The Yakutat Bay Region, Alaska, Professional Paper 64, U, S. Geol. Survey, 1909, p. 87; Tarrf R. S., Some Phenomena of the Glacier Margins in the Yakutat Bay Region, Alaska, Zeitschrift far Gletscher-kunde, Band IQ, 1908, pp. 85-88.
• Russell, I. C., 13th Ann. Rept., U. S. Geol. Survey, 1894, Pis. SHI, XIV.